Do not request data L1 buffers on scan prefetch.

Set ARC_FLAG_NO_BUF when prefetching data L1 buffers for scan. We
do not prefetch data L0 buffers, so we do not need the L1 buffers,
only want them to be ready in ARC. This saves some CPU time on the
buffers decompression.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15029